The present invention relates to the fabrication of integrated circuits. More particularly, the invention provides a technique, including a method and apparatus, for improving the quality of films deposited in a substrate processing chamber. The present invention is particularly useful for improving the quality of undoped silicon oxide films deposited by chemical vapor deposition processing, but may also be applied to other types of films (e.g., fluorsilicate glass [FSG] films) and to films deposited by other deposition techniques.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions produce a desired film. Another CVD method of depositing layers includes plasma enhanced CVD (PECVD) techniques. Plasma CVD techniques promote excitation and/or dissociation of the reactant gases by the application of energy, such as radio frequency (RF) energy, to excite the reactant gases, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the required temperature for such CVD processes. The relatively low temperature of a PECVD process makes such processes ideal for the formation of insulating layers over deposited metal layers and for the formation of other insulating layers.
One particular application of thermal CVD and PECVD techniques is to deposit silicon oxide layers over a semiconductor substrate. Because of their relatively low dielectric constant and good gap-fill properties, silicon oxide layers, including undoped silicate glass (USG), fluorosilicate glass (FSG) and others, are commonly used as insulation layers between deposited metal lines.
One particular method of depositing a USG layer includes forming a plasma from a process gas that includes tetraethoxysilane (TEOS) and oxygen. An example of such a silicon oxide film is described in U.S. Pat. No. 5,000,113, entitled "Thermal CVD/PECVD Reactor and Use for Thermal Chemical Vapor Deposition of Silicon Oxide and In-Situ Multi-step Planarized Process," issued to Applied Materials, Inc., the assignee of the present invention.
During CVD of silicon oxide and other layers onto the surface of a substrate, the deposition gases released inside the processing chamber cause unwanted deposition on areas such as the walls of the processing chamber. Unless removed, this unwanted deposition is a source of contaminate particles that may interfere with subsequent processing steps and adversely effect wafer yield.
To avoid such problems, the inside surface of the chamber is regularly cleaned to remove the unwanted deposition material from the chamber walls and similar areas of the processing chamber. This procedure is performed as a standard chamber dry clean operation where an etchant gas, such as nitrogen trifluorine (NF.sub.3), is used to remove (etch) the deposited material from the chamber walls and other areas. The etchant gas is introduced into the chamber and a plasma is formed so that the etchant gas reacts with and removes the deposited material from the chamber walls. Such cleaning procedures are commonly performed between deposition steps for every wafer or every n wafers.
The clean step can, in itself, be a source of particle accumulation, however. Fluorine from the clean plasma can be absorbed and/or trapped in the chamber walls and in other areas of the chamber such as areas that include ceramic lining or other insulation material. The trapped fluorine can be released during subsequent processing steps (e.g., by reacting with constituents from the plasma in a PECVD step) and can be absorbed in subsequently deposited silicon oxide or other layers.
To prevent such fluorine absorption, a CVD chamber is often "seasoned" after the dry clean operation. Such seasoning includes depositing a thin silicon oxide layer over the chamber walls before a substrate is introduced into the chamber. The deposited silicon oxide layer covers the absorbed fluorine reducing the likelihood that it will contaminate subsequent processing steps. After deposition of the seasoning layer is complete, the chamber is used for one to n substrate deposition steps before being cleaned by another clean operation as described above and then reseasoned.
During deposition of the seasoning layer, some of the absorbed fluorine reacts with the deposition gases and becomes bonded within the seasoning layer instead of being covered by the layer. This fluorine can then later be released from the seasoning layer during deposition of a silicon oxide or other layer over the substrate. When released during such a deposition step, the fluorine may become an unwanted contaminant that is absorbed in the deposited film. Thus, while the seasoning layer reduces the amount of fluorine absorbed into subsequently deposited films, it may not completely prevent fluorine from being absorbed into the deposited film. If maintained within acceptable levels, however, the released fluorine is not an unacceptable contaminate and should not reduce yield to unacceptable levels.
The amount of fluorine absorbed in a given layer depends in part on how many substrates are processed between each clean step. For example, if three substrates are processed between clean steps, the amount of fluorine absorbed in the layer deposited over the first substrate processed after the seasoning step may be greater than the amount of fluorine absorbed into the layer deposited over the second or third substrate processed. A x-ray photoelectron spectroscopy (XPS) graph showing the fluorine concentration in three such deposited films as a function of depth is shown in FIG. 1 to illustrate this effect. As shown in FIG. 1, the fluorine concentration in the silicon oxide layer deposited over the first substrate. (line 2) is greater than the fluorine concentration of the silicon oxide layers deposited over the second and third substrates (lines 4 and 6, respectively).
In FIG. 1, a spike 8 near the silicon/silicon oxide interface of the first substrate indicates an even higher fluorine concentration at that interface than at the corresponding silicon/silicon oxide interfaces in the films deposited over the second and third substrates. It is believed that the high fluorine concentration in spike 8 is due to a certain amount of loosely bonded fluorine present in the seasoning layer. It is believed that this loosely bonded fluorine is released from the seasoning layer during the early portion of the deposition step (e.g. during formation of the plasma in a PECVD step) of the film deposited over the first substrate. It is further believed that areas of similar fluorine concentration are not present in the latter portion of the first film or in films deposited over subsequent substrates because much of the loosely bonded fluorine is no longer available for release during deposition of these films. It is also believed that the fluorine concentration in the area of spike 8 is a source of loosely bonded fluorine in the lattice structure of the film. The loosely bonded fluorine atoms may result in the film having a tendency to absorb moisture. The absorbed moisture may increase the film's dielectric constant and can cause further problems when the film is exposed to a thermal process such as an anneal process. The high temperature of a thermal process can move the absorbed water molecules and loosely bonded fluorine atoms out of the oxide layer through metal or other subsequently deposited layers. The excursion of molecules and atoms in this manner is referred to as outgassing.
One particular method of forming a seasoning layer that has been used in the past includes introducing TEOS, oxygen and helium into the chamber and then forming a plasma from the introduced gases. When used in a resistively-heated DxZ chamber manufactured by Applied Materials, a recommended process for this seasoning layer includes introducing TEOS into the chamber at a rate of 800 mgm and combining it with helium carrier gas introduced at 560 sccm. Oxygen is introduced into the chamber at 840 sccm. A plasma is then formed by application of mixed frequency RF power. A high frequency component (13.56 MHz) is driven at 510 W, and a low frequency component (350 KHz) is driven at 130 W. The temperature of the chamber is kept at 400.degree. C., the pressure is maintained at 5 torr and the distance between the susceptor and the manifold is set to 280 mils.
The above seasoning process has successfully reduced contamination from absorbed fluorine to acceptable levels in many different applications and allowed for the deposition of stable silicon oxide films for those applications. As semiconductor device geometries continue to decrease in size, however, improved contamination reduction techniques are required for some applications. For example, a process that reduces contaminants to acceptable levels for producing integrated circuits having 0.35 .mu.m feature sizes may not be provide acceptable results for integrated circuits having 0.18 .mu.m micron feature sizes.
Thus, as device sizes become smaller and integration density increases, new and improved methods of reducing contaminants are desirable. Accordingly, it is desirable to develop improved methods of preventing fluorine absorbed in interior walls and/or ceramic lining areas of a substrate processing chamber from becoming contaminants in a deposition step performed within the chamber and from reducing the yield of subsequent processing steps.